acta chemica iasi aromatization of industrial … of industrial feedstock mainly with butanes and...

ACTA CHEMICA IASI, 17, 5-34 (2009)

Aromatization of industrial feedstock mainly with

butanes and butenes over HZSM-5 and Zn/HZSM-5

catalysts

Iuliean V. Asaftei,a* Nicolae Bilba,a Lucian M. Birsa,a and

Gheoghe Iofceab a Department of Chemistry, “Al.I. Cuza” University Iasi, 11 Carol I Bd, Iasi

700506, Romania b S. C. “Carom” S.A. , 5450, 1 Industriilor Street, Onesti, Romania

Abstract: The activity and selectivity in the aromatization of C4/C4

= hydrocarbons

on Zn/HZSM-5 (wt% 1.39 Zn) prepared by ion exchange compared with that on

HZSM-5 was investigated. The experimental results show that the conversion of

hydrocarbons and the selectivity to aromatics BTX on the bifunctional Zn/HZSM-

5 catalyst are higher than that on monofunctional HZSM-5. On HZSM-5, protons

(Brönsted acid sites) and on Zn/HZSM-5, both Zn cations and protons intervene in

alkane dehydrogenation and further in dehydrocyclooligomerization reactions that

form C6-C9 aromatics. In the liquid product, the aromatics represent an average

value of 30 wt%, with more xylenes, on HZSM-5 catalyst after 24 h time on

stream at 450 ºC and 8 atm, and 70 wt% with more toluene, on Zn/HZSM-5

catalyst, after 52 h time on stream at 450 ºC and atmospheric pressure. The

aromatization process is accompanied by oligomerization (more C9-C10 and >C10

aliphatic hydrocarbons) especially on HZSM-5 catalyst.

Keywords: Aromatization; Butanes; Butenes; Zinc exchanged HZSM-5.

_____________________

* Dr. Iuliean-Vasile Asaftei, tel.+40 232 201384, e-mail: [email protected]

6 I.V. Asaftei and co-workers

Introduction

The transformation of light hydrocarbons C2-C4 (alkanes, alkenes

and their mixtures) into high value aromatic – rich liquid hydrocarbons by

direct catalytic route1 is an area of great industrial relevance and also of

academic interest for the production of benzene, toluene and xylenes (BTX).

The main reason for the production of aromatics is the application of

aromatics as high octane blending components for gasoline (application that

tends to decrease because of carcinogenic nature of benzene, especially) and

the use as a base chemical in a number of petrochemical and chemical

processes.2

The aromatic hydrocarbons are produced from coal by coking and

from crude oil by catalytic reforming or hydroreforming of heavy naphtha,

by naphtha pyrolysis and by catalytic cracking FCC.3,4 The aromatization of

light alkanes contained in non-associated natural gas, in associated gas (as

petroleum casing-head gas) and from petroleum refining processes (as

liquefied petroleum gas, LPG) represents a new attractive way of producing

BTX aromatics. The conversion of light alkanes into aromatics (with low

selectivity to BTX) was first described in 1970 by Csicsery,5 using

bifunctional catalysts such as Pt on alumina or metal oxide on alumina.

Aromatization of light hydrocarbons into aromatics over zeolite

catalysts, in particular on middle-porous ZSM-5 (MFI) has attracted much

attention in the past decades.6–20 The properties that make ZSM-5 critical for

industrial applications are its high thermal and acid stability, high

selectivity, high activity and coke resistance in many catalytic conversions.

The activity is mostly determined by the zeolite Brönsted acid sites and by

the active metal-phase supported by zeolite and selectivity is due to the

zeolite micropores and/or cavities size and shape. The 3-D structure of

Aromatization of industrial feedstock …. 7

ZSM-5 zeolite is characterized by two dimensional types of intersecting

channels (2-D pore system) with 10-member ring (MR) openings: one type

is sinusoidal (zigzag) with near-circular (0.53× 0.56 nm) openings and the

other one is straight with elliptical (0.51× 0.55 nm) openings.21,22 The

channel intersections have a diameter of 0.89 nm and are the locus of strong

acid sites and of aromatic C6-C8 hydrocarbons formation.

Due to the shape-selective properties of the ZSM-5 framework

(determined by product shape selectivity or/and transition state selectivity),

mainly small aromatics (BTX) are formed and activation on account of coke

deposition is relatively slow, because no appreciable amounts of

polyaromatics can be formed.23

A few commercial processes24 have already been announced based

on HZSM-5 (M2 Forming process – Mobil Oil25 and M-Forming process –

Mobil26), on Ga/HZSM-5 (Cyclar-BP/UOP27-31 – Cyclization of light

hydrocarbons to aromatics - and Z-Forming from Mitsubishi Oil and

Chiyoda32), on Zn/HZSM-5 (Alpha process of Asahi Chemical and

Petrochemical33), on monofunctional Pt/K(Ba)L (AromaxTM process –

Chevron – Phillips Chemical Co.34 and RZ- Platforming process – UOP35,36

and Aroforming from IFP and Salutec based on metal oxides-HZSM-5.37,38

ZSM-5 (MFI) zeolite was discovered in the late 1965’s by Mobil

researchers39 and was patented in 1972.40 Many studies have focused on the

ability of the monofunctional acid catalyst (HZSM-5) to convert light

hydrocarbons to BTX.7-10,14-16,19,20,25,26,41,42,45-48 HZSM-5 catalysts are not the

best dehydrogenating catalysts because the hydrogen rejection from catalyst

occurs by hydrogen transfer to olefins which limits the aromatics selectivity

(≤ 30%). Aromatization over HZSM-5 is accompanied by substantial


cracking of C-C bond of alkanes with a production of 3 moles of small

alkanes per one mole of aromatics.

The enhancement of light alkanes conversion to aromatics has been

demonstrated by using metal-containing HZSM-5 catalysts instead of

HZSM-5. The catalysts investigated contain as dehydrogenating metal:

Pt;9,11,15,34-37,63,67 Ga;7,9-12,14-20,23,27-32,41,44-48 Zn;7,10,14-19,33,40-46,49-66 Pd, Ge, Ni,

Fe, Cu, Mo, Co, Ru, Re, Ag; two metals: Pt-Sn, Pt-Re, Pt-Ge, Pt-Ga, Pt-Zn,

Zn-Ni, Zn-Cu, Zn-Ga, Zn-In, Zn-Na; oxides MO (M=Zn, Cu, Ni); oxides

MO2 (M=Sn, Zr, Ce),60 oxides M2O3 (M=Ga, Fe, Cr, La, Y, In, Nd, Tl).61,62

The catalytically active species (metal, metal ion, metal oxide)

facilitate the heterolytic cleavage of the C-H bond of the adsorbed alkane

(dehydrogenation of alkanes) and accelerate the combination of surface

hydrogen atoms formed via the dehydrogenation and dehydrocyclization

process and their removal as molecular hydrogen.

Particular attention was directed to the utilization of Pt, Ga and Zn

for the aromatization of light alkanes, especially in the combination with

HZSM-5. Pt exhibits great dehydrogenation activity of light alkanes, but is,

also, an active hydrogenolysis catalyst of oligomers and cyclic compounds

and is expensive. Ga has the advantage of the lower volatility under reduced

atmosphere at high temperature and of the moderate or low activity in

hydrogenolysis. The poisonousness and the high price of gallium salt have a

limiting role. Zn ion in the di-cation state, Zn2+, with a closer d10 electronic

configuration exhibits a good dehydrogenation activity and aromatization

selectivity, seems to be stable in cationic exchange position (not in the form

of ZnO) and remains unreduced under the alkane conversion conditions.

The use of a Zn-based HZSM-5 catalyst instead of Ga-based HZSM-

5 catalyst might be preferential from environmental point of view (is a low-


waste catalyst) and due to technological (profitable) and economical

(cheaper) aspects.

The dehydrogenation metal may be incorporated into the HZSM-5

zeolite structure by aqueous ion exchange using solution of salts, by solid-

state ion exchange involving thermal treatment, chemical vapor deposition,

sublimating volatile compounds onto zeolites, mechanical mixing of metal

oxide with HZSM-5 zeolite and by isomorphous substitution in the

framework during the hydrothermal synthesis.

The purpose of this study is to develop, characterize and test the

performances of the fresh and reactivated zinc ion exchanged HZSM-5

zeolite catalyst for the conversion of butanes-butenes mixture to light

aromatic hydrocarbons. The objectives of this work are to study the catalytic

activity and selectivity of Zn/HZSM-5 catalyst, in which the NaZSM-5

zeolite was synthesized from alkaline media in presence of ethylene glycol,

the role of butenes in co-aromatization of small alkanes and the effect of

time on stream (TOS) on conversion of butanes-butenes mixture under

fixed-bed down-flow conditions at 450 ºC, 8 atm and atmospheric total

pressure and at a space velocity of 1 h-1.

Experimental

NaZSM-5 zeolite synthesis

The reactant materials used in hydrothermal synthesis were sodium

silicate solution (29.63 wt% SiO2, 9.55 wt% Na2O, 60.82 wt% H2O),

aluminum sulphate, Al2(SO4)3·18H2O (15 wt% Al2O3), sulphuric acid (96

wt% H2SO4), ammonium hydroxide solution (25 wt% NH3), ethylene glycol

and demineralized water.


NaZSM-5 zeolite used in this work was synthesized according to a

method described in the patent.70 The NaZSM-5 zeolite having SiO2/Al2O3

molar ratio of 36.02 was crystallized from a hydrogel with molar

composition of 6.49 Na2O-Al2O3-58.92 SiO2-29.43 EG-1832 H2O having a

molar ratio HO-free/SiO2=0.21. HO-

free are the HO- ions not neutralized by H+

ions added indirectly by means of aluminum sulphate and directly by means

of sulphuric acid. The synthesis run was carried out at 180 ± 5 ºC for 24 h in

a Teflon-lined stainless steel autoclave under intermittent stirring and

autogeneous pressure. The solid material was filtered, washed with

demineralized water, dried at 110 ºC for 6 h and heated in air at 550 ºC for 6

h to burn off the organic additive and to calcine.

The calcined sample was characterized by X-ray diffraction (XRD)

to identify the zeolite structure type, phase purity, degree of crystallinity and

crystallite size. The XRD measurements were performed using a Philips PW

1830 computerized-diffractometer with CuKα radiation and Ni filter with a

scan rate of 0.02º s-1 in the 2θ range of 6-30º. The surface morphology and

the size of the crystals of calcined NaZSM-5 zeolite were studied by SEM

(Microspec WDX-2A) using a 30 kV accelerating potential. The chemical

composition of the calcined NaZSM-5 sample was analyzed by the wet

method after dissolving in HF solution: aluminum concentration (Al2O3)

was determined by the chelatometric titration using EDTA and Zn standard

solution, sodium concentration (Na2O) by flame photometry using a Karl

Zeiss Jena Phlamenphotometer and SiO2 by difference between the calcined

weight of sample at 950 ºC and weight of calcined residuum at 950 ºC after

the treatment with HF solution.


Catalyst Zn/HZSM-5 preparation

First, the ammonium form, NH4ZSM-5 was obtained by ion

exchange of NaZSM-5 zeolite, three times under stirring, with 1M NH4NO3

solution (solid (g) : solution (mL) ratio of 1:5) at 80 ºC for 3 h each time.

The NH4ZSM-5 was filtered, washed, dried at 110 ºC for 6 h and calcined in

air at 550 ºC for 6 h to get protonic HZSM-5 zeolite. The HZSM-5 zeolite

was converted in the Zn/HZSM-5 form by treating it with 0.1 M Zn(NO3)2

aqueous solution (solid : solution = 1g : 5 mL) two times under stirring at 80

ºC for 6 h each time. The Zn/HZSM-5 sample was washed, dried at 110 ºC

for 6 h and calcined in air at 450 ºC for 6 h.

The zinc contained in the sample was 1.39 wt% (1.73 wt% as ZnO).

The structure of HZSM-5 and Zn/HZSM-5 samples was checked up by X-

ray diffraction, the specific surface area was determined applying the BET

method using a Carlo-Erba Sorptomatic Series 1800 instrument at -196 ºC

and the total acidity (Brönsted and Lewis) of HZSM-5 and Zn/HZSM-5

catalysts by ammonia TPD method.

The catalyst granules were obtained by extrusion of the mixture of

HZSM-5 and Zn/HZSM-5 powder with 20 wt% γ-Al2O3. Before used,

HZSM-5 was activated in air at 550 ºC for 6 h and Zn/HZSM-5 catalyst in

nitrogen at 475 ºC for 6 h.

Catalytic experiments

The catalytic experiments were carried out in a continuous flow

fixed-bed stainless-steel reactor (Twin Reactors System Naki) (with 25 mm

internal diameter) at 8 atm pressure (over HZSM-5) and at atmospheric

pressure (over ZnHZSM-5) at 450 ºC and a space velocity (WHSV) of 1 h-1

without recirculation of resulted gaseous products. The catalyst charge was

100 cm3 and the time of stream was 24 h for HZSM-5 (five tests) and 52 h


for Zn/HZSM-5 (ten tests). After each test, the catalyst was regenerated at

475 ºC for 8 h in nitrogen with 2% oxygen flow. The effluents from the

reactor were cooled in a condenser and the liquid and the gaseous fractions

collected after 4 h were analyzed by gas chromatography with a GC Carlo

Erba Mega using a fused silica capillary column (25 m length and 0.32 mm

i.d.) with SEE 52 stationary phase and FID for liquid phase, and a GC Carlo

Erba Model C, using a column (6 m length) with squalane and dimethyl

sulpholane for gaseous phase.

Results and discussion

X-ray diffraction analysis

The diffraction pattern of the starting pentasil NaZSM-5 with

SiO2/Al2O3 = 36.02 in the region of 2θ = 6 - 30º is presented in Figure 1.

From the diffractogram, the positions of diffraction lines (2θhkl), the

calculated dhkl values (nλ=2dsinθ) and the corresponding relative intensities

(I/I0) agree very well with the reported values for ZSM-5 (MFI) zeolite.22,71

The zeolite has a high degree of crystallinity (99.6%) derived from

the high intensities of the XRD lines in the region 22.5 – 25º 2θ, based on a

standard ZSM-5 crystallized in presence of tetrapropyl ammonium

hydroxide:

100standardIsampleI

ity(%)Crystallinhkl

hkl

∑∑

−

−=


Figure 1. Powder X-ray diffraction pattern and SEM micrograph of

calcined NaZSM-5.

No other diffraction peaks were found in this region (6-300 2θ),

which means the pure ZSM-5 was obtained.

The XRD patterns of HZSM-5 and of zinc-exchanged HZSM-5 (not

presented) are similar to NaZSM-5; the ion exchange process and thermal

treatment are not producing any damage to the zeolite network. The

Zn/HZSM-5 sample exhibits no diffraction peaks at 2θ = 31.60º, 34.2º,

36.1º and 56.6º which are characteristic for ZnO, indicating that the zinc is

present as Zn2+ highly dispersed in the zeolite matrix.

Morphology of the NaZSM-5 sample

The scanning electron micrograph of the NaZSM-5 sample is shown

in Figure 1. It can be seen the well developed hexagonal-shaped crystals

with crystal size of 3.5 - 4.3 μm in length and 2.1 – 2.86 μm in width and

that no crystalline material is absent.

Elemental analysis and specific surface area

The chemical composition of calcined NaZSM-5 zeolite as well as

texture parameters are presented in Table 1.


Table 1. The chemical composition of calcined NaZSM-5 zeolite and

texture parameters.

Oxidic composition, wt%

SiO2

Al2O3

Na2O

SiO2/ Al2O3 molar ratio

Zn/ZnO

NaZSM-5 92.92 4.38 2.70 36.02 - ZnHZSM-5 - 36.02 1.39/1.73 Specific surface area, m2g-1

NaZSM-5 302.5

HZSM-5 291

ZnHZSM-5 273

Static adsorption capacity after 24 h NaZSM-5

H2O 11.75

Benzene

12.7

n-hexane

10.24

Acidity and strength distribution

The acidity of the HZSM-5 and Zn/HZSM-5 catalysts was evaluated

by using temperature programmated desorption (TPD) of ammonia

technique and the NH3-spectra TPD (amount of ammonia desorbed,

mmol/g, vs temperature of desorption, ºC). Being a small molecule,

ammonia (kinetic diameter 2.62 Å) can reach all the acid sites in a zeolite.

The total amount of acid sites (Brönsted protic and Lewis aprotic acid sites)

and the acid strength (expressed as the maximum temperature of ammonia

desorption) are summarized in Table 2.

Table 2. Quantitative results of ammonia desorbed (total acidity) and

the maximum temperature of desorption (acid strength).

Desorbed ammonia, mmol/g Tmax, ºC Catalyst 1st peak

(LT) 2nd peak

(HT) Total

acidity 1st peak 2nd peak

HZSM-5 Zn/HZSM-5

0.618 0.638

0.282 0.144

0.900 0.782

220 200

420 420

Both catalysts give two humps (peaks), one at low temperature (LT)

(T < 300 ºC) and the other one at high temperature (HT) (T > 300 ºC). The


low region with Tmax = 200 – 220 ºC corresponds to the low strength of

Lewis and weak Brönsted acid sites and consists of the physisorbed

ammonia (ammonia is coordinatively bonded to the terminal silanol

groups). The high region with Tmax = 420 ºC corresponds to the moderate

and high strength of Brönsted acid sites and consists of the chemisorbed

ammonia (ammonia forms ammonium ions hydrogen-bonded to oxygen of

the framework).

Cation exchange of NaZSM-5 with protons (H+ ions) forms

hydroxyl groups resulting in a bridging ≡ Si-OH-Al ≡ structure. These

groups known as Brönsted acid sites are in a dissociation equilibrium, like

mineral acids in solution:72

Cation exchange of HZSM-5 with multivalent cations or transition

metal ions (Zn2+) forms weak and moderate strong acid sites, especially

Lewis acid sites. Lewis acid sites can be also generated by dehydration from

two acidic hydroxyl groups at temperature higher than 480 ºC, by presence

of hydroxyl nests.73

The total acidity of ZSM-5 catalysts is due to the number of acid

sites (proportional to the Al content of framework) and to the strength of


acid sites (determined by the number of Al atoms that are adjacent to a

proton as second- nearest neighbors). The sites (protons) corresponding to

framework bridged hydroxyl groups surrounded by less than three-nearest

neighbors are moderate acid sites and are mostly Brönsted acid sites. The

sites corresponding to protons surrounded by three Al atoms as second-

nearest neighbors are weak acid sites and are mostly of Lewis type. Each Al

atom has four Si atoms as nearest neighbors (Lovenstein’s rule). The

catalytic activity of zeolite catalyst takes into consideration the total number

of acid sites, the ratio of Brönsted to Lewis sites and the acid strength

distribution of each types of site.

Catalytic performance of HZSM-5 catalyst in C4/ C4= hydrocarbons

aromatization

Table 3 presents the feedstock composition and reaction conditions

of aromatization for five cycles (with intermediate regeneration) on HZSM-

5 catalyst.

Table 3. The butanes-butenes feedstock composition and conditions

of aromatization on HZSM-5 catalyst.

Test number Feedstock composition %(vol) 1 2 3 4 5

C2 0.015 0.015 0.03 - - C3 0.76 0.764 1.07 1.11 -

i-C4 22.46 22.46 35.39 23.84 27.49 n-C4 27.20 27.20 26.85 32.69 34.31 i-C5 0.03 0.03 0.02 0.25 0.29

1-C4= 29.15 29.15 22.13 22.33 19.02

i-C4= 0.39 0.30 0.12 0.07 -

tr.-2-C4= 11.45 11.45 8.90 12.66 11.20

cis-2-C4= 6.62 6.62 4.65 6.57 6.09

1,3-C4= = 1.73 1.73 0.58 0.33 0.31

Time on stream, h 30 24 20 24 28


Temperature, ºC 415 450 450 450 450 WHSV, h-1 1 1 1.2 1 1

Pressure, atm 8 8 8 8 8 Catalyst, cm3 100 100 100 100 100

The performances of the HZSM-5 catalyst during the test

no.4 corresponding to each 4 h time on stream are presented in Table 4 and

Table 5.

Table 4. Gaseous hydrocarbons (HC) distribution over HZSM-5 catalyst.

Time on stream, h HC (vol.%) 4 8 12 16 20 24

C1 + C2 29.50 12.71 4.21 1.14 0.46 0.13 C2

= 0.14 0.07 0.32 0.55 0.61 0.24 C3 (1.11) 59.68 61.68 37.73 13.61 5.72 2.98 i-C4 (23.84) 3.77 10.39 23.83 35.28 32.21 27.56 n-C4 (32.69) 5.39 12.05 27.07 39.97 45.46 46.00 i-C5 0.53 1.56 2.58 1.61 0.97 0.35 1-C4

= (22.33) 0.11 0.09 0.25 2.03 3.41 6.76 i-C4

= (0.07) 0.46 0.89 2.23 3.05 3.99 2.83 tr 2-C4

= (12.66) 0.14 0.14 0.51 1.25 3.76 5.57 cis-2-C4

= (6.57) 0.13 0.11 0.33 0.92 2.64 7.45 1,3-C4

= = (0.33) 0.02 0.08 0.31 0.11 0.28 0.20 In parenthesis: concentration in feedstock no.4

Table 5. Liquid hydrocarbons (HC) distribution over HZSM-5 catalyst

Time on stream, h HC (vol.%) 4 8 12 16 20 24

i-C5 2.02 5.61 12.08 16.15 16.10 11.78 n-C5 0.19 0.93 2.77 1.62 1.08 0.92 i-C6 - 0.66 3.43 8.05 10.47 6.94 n-C6 0.10 0.26 0.84 0.64 1.33 0.66 i-C7 0.08 0.62 5.04 13.42 14.10 9.42 B 7.29 7.08 2.84 0.76 1.95 0.45 n-C8 - 0.23 0.76 1.22 1.42 0.91 n-C7 - 0.10 1.14 5.11 7.00 5.53 i-C8 - 0.21 1.66 4.94 7.73 10.94 T 24.00 20.92 16.16 7.36 5.83 5.68


i-C9 - 0.26 1.33 7.39 12.39 21.34 EB 0.14 0.07 0.09 0.90 1.46 1.84 X 20.08 20.29 21.00 10.84 5.84 5.44 n-C9 5.30 4.48 3.71 3.05 2.41 2.51 IPB - 0.23 0.01 0.52 0.53 1.27 i-C10 0.09 - 0.30 1.27 1.62 1.93 n-C10 4.18 6.47 7.23 5.48 2.86 2.70 > C10 36.28 31.59 19.29 11.04 5.54 9.45 Σ Ar 51.51 48.36 40.09 19.86 15.60 14.98 C5 – C10 11.96 19.74 39.98 68.35 78.42 75.28 B–Benzene; T–Toluene; X-Xylene, EB–Ethyl Benzene; IPB–iso propyl benzene

Conversion of mixed butanes/butenes feedstock no.4 to aromatics

over HZSM-5 takes place with low selectivity to BTX, the reactants

forming predominantly the cracking products.

The changes in the gaseous product distribution and of liquid phase

over HZSM-5 with time on-stream in the transformation of butanes and

butenes are presented in Figure 2.

0

20

40

60

80

100

0 4 8 12 16 20 24Time on stream, h

gase

ous

prod

uct,

vol.%

0

20

40

60

80

100

0 4 8 12 16 20 24Time on stream, h

liqui

d pr

oduc

t, w

t.%

◆-Σ C4; ■-Σ C4=; ▲-C1+C2; ⋇-C3 ◆- Σ Ar ; ■-C5–C10 ; ▲-> C10; ⋇-B;

○-T; x-X+EB Figure 2. Gaseous phase composition and liquid phase composition vs time

on stream over HZSM-5 catalyst in butane/butenes conversion.


The concentration of butanes (n+i) decreased from 56.53% to 9.16%

in the first 4 h of reaction, after that is continuously increasing going beyond

the feedstock after 16 h. The concentration of butenes (1-C4=, trans-2-C4

=

and cis-2-C4=) decreased from 41.63% to 0.84% in the first 4 h, and to

3.32% after 12 h of reaction. The forming of methane and ethane (C1 + C2)

reach the maximum value (29.50%) after 4 h of reaction and the forming of

propane (C3) after 8 h of reaction (61.68%); their production is connected to

aromatic hydrocarbons formation. The molecular hydrogen was not detected

in the gaseous phase.

The catalytic activity and selectivity to aromatic hydrocarbons is

visible for first 12 h run; after that aliphatic C5 – C10 are formed.

Catalytic performance of Zn/HZSM-5 catalyst in C4/C4=

hydrocarbons aromatization

The industrially feedstock composition and the conditions of

aromatization on ion exchanged Zn/HZSM-5 during of ten catalytic tests are

given in Table 6.

Table 6. The butanes-butenes feedstock composition and conditions of

aromatization on Zn/HZSM-5 catalyst. Test number Feedstock

composition (vol.%)

1 2 3 4 5 6 7 8 9 10

C3 1.08 0.28 0.89 0.62 0.42 0.24 0.26 0.31 0.30 0.57

n-C4 9.67 2.90 5.72 5.40 12.02 10.58 11.42 12.52 11.36 14.63

i-C4 57.34 49.34 51.91 52.53 44.85 36.10 31.33 33.21 35.24 38.52

1-C4= 9.78 16.20 10.57 16.97 12.59 16.07 17.90 16.53 15.85 13.39

i-C4= 11.03 26.28 26.00 19.34 24.39 30.33 31.78 29.42 30.16 12.63

tr.-2-C4= 6.72 2.77 3.15 3.54 4.42 4.64 5.16 5.61 5.23 11.86

cis-2-C4= 3.34 1.71 1.64 1.35 1.20 2.05 2.12 2.36 1.84 7.53

TOS : 52 h; temperature: 450 ºC; atmospheric pressure; WHSV=1 h-1; catalyst; 100 cm3; regeneration of catalyst after each test: 475 ºC for 6h in nitrogen flow with 2% oxygen.


The performances of the Zn/HZSM-5 catalyst during the catalytic

test no.1 are presented in Table 7 and 8.

Table 7. Gaseous hydrocarbons (HC) distribution over Zn/HZSM-5

catalyst.

Time on-stream, h HC vol.% 4 8 12 16 20 24 28 32 36 40 44 48 52 C2 2.38 4.81 2.98 2.37 1.62 1.87 1.99 1.25 2.47 1.94 1.86 2.24 1.37

C2= 1.87 1.05 1.41 1.58 1.24 1.16 1.22 0.69 1.46 1.49 1.41 1.93 1.85

C3 22.36 25.21 20.48 20.26 16.20 15.73 17.25 23.21 21.76 17.49 15.01 18.00 12.68

i-C4 14.5 8.03 15.2 18.07 18.91 19.83 22.48 21.69 25.39 27.33 28.53 25.52 28.84

n-C4 7.24 7.0 8.03 8.16 7.59 8.22 8.96 10.0 10.1 11.9 12.87 11.68 10.43

i-C5 0.67 0.49 0.46 0.66 0.68 0.61 0.78 0.76 0.71 0.98 1.04 0.89 0.80

1-C4= 0.16 0.26 0.23 0.18 0.16 0.11 0.13 0.15 0.17 0.29 0.16 0.21 0.21

i-C4= 0.62 0.80 0.73 0.78 0.73 0.61 0.78 0.77 0.78 1.83 1.25 1.16 1.10

tr-2-C4

= 0.25 0.27 0.31 0.27 0.24 0.24 0.26 0.27 0.33 0.44 0.50 0.45 0.38

cis2-C4

= 0.19 0.23 0.25 0.20 0.30 0.01 0.19 0.20 0.24 0.28 0.36 0.25 0.30

H2 50.0 51.43 49.52 47.14 52.09 51.47 45.50 40.95 36.20 36.36 36.57 37.75 42.45

Table 8. Liquid hydrocarbons (HC) distribution over Zn/HZSM-5.

Time on-stream, h HC

vol.% 4 8 12 16 20 24 28 32 36 40 44 48 52

i-C5 1.18 2.34 5.36 3.00 4.57 5.07 5.81 6.88 7.65 7.57 8.0 7.16 6.93

i-C6 0.14 0.11 0.29 0.50 0.62 0.48 0.51 0.42 0.55 1.58 1.67 1.69 1.51

i-C7 0.07 0.05 0.01 0.24 0.31 0.38 0.37 0.71 1.08 1.50 1.37 1.29 1.16

B 9.53 13.0 12.48 11.93 10.99 9.42 9.22 9.19 9.31 7.89 7.67 7.55 6.80

i-C8 - 0.02 0.06 0.06 0.07 0.05 0.05 0.09 0.16 0.20 0.33 0.32 0.30

T 43.71 43.03 41.36 42.89 42.75 43.26 43.06 41.59 40.02 39.84 39.50 39.18 39.24

X+EB 22.77 22.37 21.43 20.24 20.89 21.41 22.13 21.10 20.72 20.88 21.21 22.13 24.31

n-C9 7.24 6.59 6.64 6.27 6.20 7.14 6.52 6.61 6.65 6.52 5.63 6.47 6.30

IPB 0.16 0.20 0.22 0.43 0.41 0.24 0.32 0.20 0.21 0.39 0.33 0.20 0.21

n-C10 4.43 4.25 4.74 4.50 4.52 5.09 4.94 5.54 5.33 5.55 5.75 6.80 6.25

i-C10 0.20 0.01 0.21 0.30 0.24 0.14 0.14 0.63 0.67 0.81 0.67 0.63 0.76


> C10 9.09 6.22 5.54 6.90 6.66 6.75 6.28 6.44 6.43 6.48 6.91 5.55 4.92

N 1.34 1.64 1.17 2.29 2.10 0.06 - - - - - 0.22 0.19

Σ Ar 76.01 78.40 75.27 75.06 74.63 74.09 74.41 71.88 70.05 68.61 68.38 68.86 70.35

C5-C10 13.26 13.37 17.31 14.87 16.53 18.35 18.34 20.88 22.09 23.73 23.42 24.36 23.31

B–Benzene; T–Toluene; X-Xylene, EB–Ethyl Benzene; N- Naphtalene Aromatization of a mixture containing 57.41% butanes and 30.87%

butenes (feedstock no.1) over Zn/HZSM-5 takes place with high selectivity

to aromatics BTX and with production of molecular hydrogen. The changes

in the gaseous product distribution over Zn/HZSM-5 with time on-stream

(from four to four hours), test no.1 in comparison with the test no.10, are

shown in Figure 3.

0

20

40

60

80

0 4 8 12 16 20 2428 32 36 40 44 48 52Time on stream, h

Gas

eous

pha

se c

ompo

sitio

n,vo

l.%

Test no.1

0

20

40

60

80

0 4 8 12 16 20 24 28 32 36 40 44 48

Time on stream, h

Test no.10

Figure 3. Gaseous phase composition vs. TOS over Zn/HZSM-5 catalyst:

◆-Σ C4; ■-Σ C4=; ▲-C2+C2

=; ⋇-C3 ; ○-H2

The concentration of butenes decreased from 30.87 vol.% to 1.22

vol.% after first 4 h of reaction and remains at values smaller than 2.0%

after 52 h of reaction. The concentration of butanes (n+i) decreased from

67.01% to 15.03% after 8 h of reaction, after that is continuously increasing


without to rise above the initial concentration. It is evident that after about

470 h of catalytic reaction and nine regeneration steps at 475 ºC the catalytic

activity and selectivity to aromatics BTX are present. The hydrogen

molecular concentration exceeds the butanes concentration during the first

40 h of reaction in test no.1 and after 24 h of reaction in test no.10. The

thermal treatments and the partial removal of coke deposited could be the

reason for this diminution but the zinc is still present in the catalyst. The

main gaseous hydrocarbon over Zn/HZSM-5 is propane (~ 20 vol%) near

by C2-C2=, more less than over HZSM-5 (about 60 vol%).

The aromatic hydrocarbon distribution in the liquid phase

corresponding to catalytic tests no.1 and no.10 is plotted in Figure 4.

0

20

40

60

80

100

0 4 8 12 16 20 24 28 32 36 40 44 48 52

Time on stream, h

Con

cent

ratio

n, w

t%

Test no.1

0

20

40

60

80

100

0 4 8 12 16 20 24 28 32 36 40 44 48

Time on stream, h

Test no.10

Figure 4. Distribution of hydrocarbons in liquid phase:

◆- Σ Ar ; ■-C5–C10 ; ▲-> C10; ⋇-B; ○-T; x-X+EB

The produced aromatic hydrocarbons were mainly toluene (~40

wt%) and xylenes (~21 wt%); the benzene in the aromatics was about 9

wt% greater than over HZSM-5. The aliphatic hydrocarbons C5-C10 fraction


in the liquid phase is increasing from ~ 13 wt% after 4 h of reaction to ~ 23

wt% after 52 h of reaction and is based on C9 and C10 hydrocarbons. The

formation of aliphatic hydrocarbons with more than 10 carbon atoms (> C10)

is limited to about 6 wt%. In the liquid product of all catalytic tests is

present naphthalene in a concentration below 1.0 wt%.

The average output of aromatic hydrocarbons BTX during the five

catalytic experiments over HZSM-5 catalyst is presented in Figure 5.

0

20

40

60

80

100

1 2 3 4 5Number of test

Con

cent

ratio

n, w

t.%

B

T

X+EBAr

Figure 5. The aromatics average output over HZSM-5 catalyst

(4500C, 8 atm., WHSV=1 h-1).

The average output of aromatics BTX over HZSM-5 monofunctional

catalyst does not go beyond 30 wt% in the liquid phase and the formation of

xylenes and toluene is of preference.

In Figure 6 is presented the average output of aromatics BTX during

the ten catalytic experiments over Zn/HZSM-5.

The average output of aromatic hydrocarbons BTX over Zn/HZSM-

5 bifunctional catalyst represents more than 70 wt% in the liquid phase in

all ten catalytic tests.


0

20

40

60

80

100

1 2 3 4 5 6 7 8 9 10

Number of test

Con

cent

ratio

n, w

t%

B

T

X+EB

Ar

Figure 6. The aromatics average output over Zn/HZSM-5 catalyst

(4500C, atm. pressure, WHSV =1 h-1).

Aromatization of multicomponent feedstock is complex, involving a

large number of heterogeneous reactions.

It is generally agreed that the aromatization of low molecular weight

alkanes can be represented as a three stage process: 1) alkanes conversion

into small alkenes, C2=- C4

=, 2) alkene oligomerization and cracking, and 3)

aromatization of small alkenes into C6-C10 aromatic hydrocarbons.

The relevant reactions consist of: 1) alkane C-H bond activation

through a pentavalent carbonium ion and of alkene C-H through a trivalent

carbenium ion (through protonation); 2) dehydrogenation of carbonium ion

to carbenium ion and finally to small alkenes, C2=- C4

=; 3) oligomerization

of small alkenes to higher alkenes, C6- C10; 4) rapid isomerization, 5) β-

scission; 6) dehydrogenation of higher alkenes to dienes; 7) cyclization of

diene to cyclic alkenes; 8) dehydrogenation of cyclic alkenes to cyclic

dialkenes and finally to aromatics, C6- C10.15,41,47,48,76


In the presence of HZSM-5 catalyst, the first stage of alkane

transformation proceeds via two routes: protolytic cracking route of C-C

and C-H bonds in the alkane molecules on strong acid sites to very unstable

carbonium ion that collapses to give alkanes (or H2) and adsorbed

carbenium ions, and hydrogen transfer route (dehydrogenation) between

the alkane with the alkenic intermediates adsorbed on the Bronsted acid

strong sites resulting new alkanes as side products.7,18,41,48,74-77 The second

stage, alkene interconversion, includes alkene isomerization,

oligomerization and cracking steps. The third stage, alkene aromatization,

proceeds via a sequence of cyclization and hydrogen transfer with formation

of aromatics and alkanes.

Monofunctional HZSM-5 catalyst exhibits preferentially high

cracking, isomerization and β-scission reactivity that lead to loss of carbon

atoms to undesirable products.14 Hydrogen rejection from surface occurs by

hydrogen transfer to alkanes which limits the aromatics yield that can be

obtained on HZSM-5.

It is well known that the acidity of the surface of the catalyst has a

decisive effect on the activity of the catalyst. The acidity is influenced by

the type of the acid sites, such as the Brönsted (protic acid sites) and Lewis

(aprotic acid sites) sites as well as the number and the strength of the acid

sites. The type and concentration of acid sites are controlled by the location

of Al atoms at the framework (AlO4)- and non-framework position. The

total amount of acid sites (Brönsted and Lewis) of the HZSM-5 and

Zn/HZSM-5 and the acid strength expressed as the maximum temperature

of ammonia desorption are summarized in Table 2. Brönsted sites can form

both carbenium and carbonium ion intermediates whereas Lewis sites

produce only carbenium ions.


The HZSM-5 catalyst contains predominantly Brönsted strong acid

sites related to aluminum located in the framework and Brönsted weak acid

sites related to some silanol groups and Lewis acid sites related to the extra-

framework aluminum (AlO+ species) or distorted aluminum in framework.

The HZSM-5 catalyst is a solid Brönsted superacid, which at high

temperature (~ 500 ºC) can even protonate alkanes. The acid strength of

HZSM-5 catalyst is enhanced by the presence of extra-framework

aluminum which is easily generated during the synthesis of ZSM-5 by non-

template method. The total acidity of HZSM-5 catalyst determined by

ammonia-TPD method has the value of 0.900 mmol/g and the high strength

(mostly Brönsted acid sites) to the region with Tmax= 420 ºC (0.282

mmol/g). The strength of the Bronsted acid sites is strong in HZSM-5

catalyst and the conversion of alkenes and alkanes to small alkenes takes

place via acid cracking and hydrogen-transfer reactions.

The total acidity of zinc ion-exchanged HZSM-5 (50%) is lower

(0.782 mmol/g) and the strength of acid sites in the region with Tmax= 420

ºC is only 0.144 mmol/g. After the zinc incorporation by ion exchange

method the concentration of Brönsted acid sites decreases and the amount of

Lewis sites increases.14,49 Zn/HZSM-5 catalyst contains medium acidity that

minimizes the occurrences of cracking reactions. The zinc incorporated in

HZSM-5 zeolite through ion exchange is very well dispersed and is stable in

isolated cationic (Zn2+) positions with tetrahedral symmetry and of ZnOH+

not thermally stable either compensating the charge of two Al tetrahedra (-

O-Zn2+-O-) or legated to one internal silanol or OH group (≡ Si-OZn).53,74 In

the case of Zn/HZSM-5 catalyst the alkanes dehydrocyclodimerization

proceeds via bifunctional pathways involving exchanged cations for

dehydrogenation of alkane and dehydrocyclization of alkenic oligomers and


acidic OH groups for alkene interconversion and aromatic formation. Zinc

cations as Lewis acid sites promote alkane dehydrogenation (heterolytic

cleavage of the C-H bond) to alkene (dehydrogenation function) and

oligomers dehydrogenation to oligomers with one or more double bonds,

decreases the β-scission rates and the residence time of alkenes within

oligomerization/ β-scission cycles,16 exerts strong hydrogen attracting action

and promotes removal of hydrogen atoms adsorbed as hydrogen molecular, 16,18,40,41 prevents hydrogenation of alkenes required in cyclization and

consequently, enhances aromatization.41,74 The ZnK-edge X-ray absorption

studies and TPR studies show that Zn2+ cations in Zn/HZSM-5 do not

reduce to zerovalent species (Zn0).14,52

Conclusions

Monofunctional acid catalyst HZSM-5 exhibits a low selectivity to

aromatics BTX in the catalytic aromatization of butanes- butenes mixture,

due to preferentially cracking, isomerization, and β-scission reactivity. The

average outputs of aromatics BTX do not go beyond 30 wt% in the liquid

phase and the formation of xylenes and toluene is of preference. In the

HZSM-5 are present Brönsted and Lewis acid sites with acidic OH groups

located at channel intersections.

Bifunctional catalyst Zn/HZSM-5 exhibits high selectivity to

aromatics BTX in the aromatization of butanes- butenes mixture, due to

dehydrogenation of alkanes to alkenes and dehydrocyclization of alkenic

oligomers to naphtenic intermediates on exchanged Zn2+ cations (Lewis

strong acid sites), and of alkene interconversion and aromatic formation on

acid OH groups (Brönsted strong acid sites). The average output of

aromatics BTX in the liquid phase represents more than 70 wt% and the


formation of toluene (~40 wt%) and xylenes (~21 wt%) are of preference.

The significant production of aromatics is explained by the enhanced

production of alkenes by the effective dehydrogenating action of zinc on

alkanes.

The high Brönsted acidity of HZSM-5 catalyst is responsible for the

high percentage of C5-C10 aliphatic hydrocarbons (~80 wt% after 24 h

TOS), compared with Zn/HZSM-5 catalyst (~25 wt% after 52 h TOS).

The product distribution (gaseous and liquid) in the conversion of

butanes- butenes mixtures at 450 ºC and atmospheric pressure over HZSM-

5 and Zn/HZSM-5 catalysts is changing with time on-stream. The HZSM-5

catalyst deactivates fast and Zn/HZSM-5 catalyst is able to sustain activity

and selectivity for a longer period.

The presence of butenes in the butanes feed exercises the activation

of butanes: it is thought that butenes are protonated to carbenium ions from

a Brönsted acid site and then activate butanes through hydride abstraction.

The catalytic aromatization reactions over Zn/HZSM-5 catalyst can

upgrade the low-value light hydrocarbon byproduct streams from refinery

and cracker operations, producing aromatics BTX and hydrogen as co-

product.

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acta chemica iasi aromatization of industrial … of industrial feedstock mainly with butanes and...

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